In a conventional image processing apparatus such as an electronic still camera, an image signal photographed and output by an image sensor is converted into digital data by an A/D converter, and undergoes necessary image processing in many cases. After that, the image signal undergoes irreversible compression conversion such as JPEG encoding, and is recorded on a recording medium.
The irreversibly compressed image data is read out from the recording medium inside the electronic still camera, decoded (reconstructed), and displayed on a display unit or the like. In this case, it is also possible to obtain histogram information indicating the frequency distribution of the luminance from the decoded playback image data, and display this histogram information together with the decoded image data on the display unit such as a liquid crystal monitor. This allows the user to determine whether the exposure level of the photographed image data is correct, or whether the photographing conditions are appropriate.
A technique which displays a bar chart or line graph representing the frequency for each luminance level on a liquid crystal monitor or the like in order to allow easy check of the correctness of the exposure level during photography is already well-known.
When irreversible compression such as JPEG encoding is performed, however, the original characteristic, e.g., the accurate luminance information of image data is lost. Accordingly, histogram information obtained from decoded image data sometimes largely differs from the histogram information of image data before irreversible compression. Especially when irreversible compression is performed at a high compression ratio, a luminance value degenerates for each quantization step during the course of quantization in the irreversible compression. Consequently, as shown in FIG. 6B, the histogram information becomes discrete.
Also, as the number of pixels of the image sensor increases, the decoding time of image data recorded at the maximum resolution prolongs. This lowers the response when the histogram is displayed together with the playback image. To prevent this lowering of the response, therefore, a thumbnail image is displayed by resizing where necessary, or a prerecorded image having resolution lower than the maximum resolution is displayed. However, when the resolution is decreased, the original characteristic, e.g., the accurate luminance information of the image data is lost. Consequently, as shown in FIG. 6C, the obtained histogram information sometimes differs from the histogram information, FIG. 6A, of image data before resolution conversion.
As described above, when histogram information obtained from image data decoded inside an electronic still camera or the like is displayed together with the image data, the displayed histogram information may be different from the actual human sense, and may also be different from the exposure conditions during photography.
FIGS. 6A to 6C explain this problem. FIG. 6A shows a histogram obtained from original image data before resolution conversion. FIG. 6B shows a histogram after irreversible compression is performed at a high compression ratio. FIG. 6C shows a histogram after conversion into a low resolution and irreversible compression are performed. As shown in FIGS. 6A to 6C, the histogram after irreversible compression is performed at a high compression ratio largely differs from the distribution curve of the histogram indicating the exposure conditions during photography.